We propose that tumor cell lysis by cytotoxic T-lymphocytes (CTL) is physiologically distinct from that mediated by antibody plus complement (Ab plus c'). The two forms of lysis can be distinguished by monitoring the fate of the tumor cell nucleus. We will extend these observations to other forms of immune lysis as a preliminary step towards elucidating the relative importance of various forms of immune protection in different anatomical sites in the whole animal. Our observations have led us to propose that CTL-initiated lysis is an autolytic event carried out by the target after receiving a signal from the CTL. Using both murine and human effector systems we will attempt to elucidate early events in the target cell after CTL attack and formulate a more precise mechanism of CTL-induced lysis. In addition we will continue our studies on a model of a tumor escape mechanism on a model with a murine tumor that undergoes cyclical changes in CTL sensitivity despite a constant expression of H-2 antigen.